Light weight head mounted image display device

ABSTRACT

An image display element  1  and an observation optical system  2  which forms an exit pupil  4  for observation of an image displayed on the image display element are included, wherein the observation optical system  2  has at least one surface  2   1  that has a lens function, and the following condition (1) is satisfied:  
     0.1&lt; P·PD·ZD &lt;5   (1)  
     where P is a pixel pitch (in μm) of the image display element, PD is a diameter (in mm) of the exit pupil, and ZD is a distance (in mm) from the display surface of the image display element to the first surface having a lens function. Whereby, weight reduction is achieved while good image quality is maintained regarding an image display apparatus that is used for magnifying observation of an image on a display element.

BACKGROUND OF THE INVENTION

[0001] 1) Field of the Invention

[0002] The present invention relates to an image display apparatushaving an observation optical system.

[0003] 2) Description of Related Art

[0004] In recent years, development has been energetically made forimage display apparatuses, specifically for those to be held on the heador face of individuals for entertaining them with a wide-screen image.Conventionally, as head-mount image display apparatuses, there are knowna type in which an image on an image display element such as a CRT istransmitted via an image transmitting element to an object surface,where the image is projected in the air by a toric reflecting surface(U.S. Pat. No. 4,026,641), and a type in which an image displayed on aliquid crystal display device (LCD) is once imaged in the air via arefraction-type relay optical system and then is introduced into an eyeof an observer via an eyepiece optical system composed of a concavemirror arranged in a decentered manner (Japanese Patent ApplicationPreliminary Publication (KOKAI) No. Hei 6-294943).

[0005] However, these types of the head-mount image display apparatusare not suitable for long-time use as being held on the head or face,because their optical systems are large and heavy. Therefore, it isdesired to reduce the weight of the apparatus while maintaining goodimage quality.

[0006] Also, in accordance with recent popularization of cellular phoneand portable intelligent terminal, requirements for wide-screen view,via an image display apparatus, of graphics or text data on a cellularphone or portable intelligent terminal have grown. For application to acellular phone or the like also, it is desired to reduce weight of theapparatus while maintaining good image quality because users wouldpersonally carry the cellular phones or the like with them in theirpockets or bags.

[0007] As conventional resolution examples for achieving weightreduction, observation optical systems that are constituted with acombination of thin platelike optical members (Japanese PatentApplication Preliminary Publication (KOKAI) No. Hei 8-234137, Japanesepatent Application Preliminary Publication (KOKAI) No. 8-240773) areknown.

[0008] Although such an observation optical system is allowed to have alightweight structure, it involves a problem in having difficulty inproviding good image quality, because dust is easily caught onreflecting surfaces thereof to cause flare or the like.

[0009] In reference to FIGS. 29-30, this problem is explained. Each ofobservation optical systems shown in FIGS. 29, 30 is configured toreflect bundles of rays emergent from an image display element 1 such asLCD at a reflecting surface 2 ₁ of an optical member 2. Since theobservation optical system as shown in FIG. 29 uses a prism, there is nochance that dust or flaws would damage the reflecting surface 2 ₁ duringassembling or transportation. In contrast, since the observation opticalsystem as shown in FIG. 30 is constituted with a combination of thinplatelike optical members for the purpose of weight reduction, thereflecting surface 2 ₁ is apt to be damaged with dust or flaws and thusassembling and transportation is difficult.

SUMMARY OF THE INVENTION

[0010] Therefore, an object of the present invention is to achieveweight reduction while maintaining good image quality in an imagedisplay apparatus that is used for magnifying observation of an imageformed on a display element.

[0011] An image display apparatus according to the first aspect of thepresent invention includes an image display element and an observationoptical system which forms an exit pupil for observation of an imagedisplayed on the image display element, wherein the observation opticalsystem has at least one surface having a lens function, and thefollowing condition (1) is satisfied:

0.1<P·PD·ZD<5   (1)

[0012] where P is a pixel pitch (in μm) of the image display element, PDis a diameter (in mm) of the exit pupil, and ZD is a distance (in mm)from the display surface of the image display element to a first surfacehaving a lens function.

[0013] For example, in the optical system provided with an opticalmember similar to that shown in FIG. 30, it is desirable that Condition(1) is satisfied for the purpose of obtaining good image quality withoptical performance being little affected by dust or flaws that woulddamage the reflecting surface during assembling or transportation. InFIG. 1, the reference numeral 1 represents an image display element suchas a LCD, the reference 2 represents a first optical member, thereference numeral 2 ₁ represents a first reflecting surface, and thereference numeral 4 represents an exit pupil.

[0014] Light emergent from the image display element 1 has a relativelysmall beam diameter as it is incident on the first reflecting surface 2₁, and the beam diameter is widened after the light is reflected fromthe reflecting surface 2 ₁. Therefore, if dust is caught on the firstreflecting surface 2 ₁, a blurred image of the dust is projected on thepupil as enlarged for observation, to degrade image quality.

[0015] In this case, under the condition where a dust particle with thesame size is caught on the reflecting surface 2 ₁, a finer pixel pitchof the image display element 1 causes the blurred image of the dustparticle to be more conspicuous because resolution of the image isfiner, while a coarser pixel pitch makes the image of the dust particleto be less conspicuous because resolution over the entire image iscoarser. Also, a larger diameter of the exit pupil 3, or a larger valueof NA causes a larger beam diameter at the first reflecting surface, andthus the dust particle is projected relatively small to be lessconspicuous. Also, a longer distance from the display surface of theimage display element 1 to the first surface having a lens functioncauses a larger beam diameter at the first reflecting surface, and thusthe dust particle is projected relatively small to be less conspicuous.

[0016] Considering the discussions set forth above collectively, theapplicant has revealed that satisfaction of Condition (1) is desirablein the image display apparatus according to the first aspect.

[0017] If Condition (1) is satisfied, it is possible to achieve goodimage quality because dust, which would be caught on the reflectingsurface during fabrication, is inconspicuous as observed.

[0018] Failing to reach the lower limit value, 0.1 of Condition (1) isnot preferable, because it degrades image quality with dust caught onthe reflecting surface being conspicuous as a large image. Exceeding theupper limit value, 5 of Condition (1) is not preferable, because such aconfiguration is impractical with bulkiness of the optical system orwith degraded image quality over the entire field.

[0019] Also, an image display apparatus according to the second aspectof the present invention includes an image display element and anobservation optical system which forms an exit pupil for observation ofan image displayed on the image display element, wherein the observationoptical system includes at least one diffraction element which is givena lens function by diffraction effect, and an optical element satisfyingthe following condition (2) is used in observation:

a<90   (2)

[0020] where a is a transmittance (in %) for a wavelength range of 500nm-650 nm.

[0021] As measures to achieve weight reduction of the observationoptical system, use of a diffraction element, which has a lens functionin spite of its thin structure, is known. As a reflection type—ortransmission type—diffraction element, a relief hologram or a volumehologram is available. However, a diffraction element involves a problemof flare caused by undesired order rays. Therefore, if light from brightlight source such as the sun or an electric lamp is incident on adiffraction element, for example, flare caused at the diffractionelement prevents a good image from being formed.

[0022] Therefore, in the image display apparatus according to the secondaspect, an optical member which satisfies Condition (2) is used for thepurpose of attenuating amount of light from the light source, whichwould be the cause of flare.

[0023] Satisfaction of Condition (2) allows good image to be obtainedwith flare by the diffraction element being reduced.

[0024] Also, an image display apparatus according to the third aspect ofthe present invention includes an image display element and anobservation optical system which forms an exit pupil for observation ofan image displayed on the image display element, wherein the observationoptical system includes a first unit having at least one prism memberwith a positive refracting power and a second unit, and the first unitis configured to be movable for alignment of optical axes.

[0025] For weight reduction of the apparatus, it is effective toseparate the optical member having a positive refracting power into afirst unit having a positive refracting power and a second unit, with aspace between the first unit and the second unit being filled with air.

[0026] However, if such a two-unit configuration is employed, theoptical axes of the units are likely to be inconsistent. Also, sincesensitivity to inconsistency of the optical axes is large, it sometimesis difficult to obtain good image quality.

[0027] Therefore, according to the third aspect, the first unitincluding at least one prism member with a positive refracting power isconfigured to be movable.

[0028] According to the configuration of the present invention, theoptical axes can be aligned, in assembling, by movement of the firstunit including the prism member, thereby to achieve good imagingperformance.

[0029] Also, an image display apparatus according to the fourth aspectof the present invention includes an image display element and anobservation optical system which forms an exit pupil for observation ofan image displayed on the image display element, wherein the observationoptical system includes a first unit that includes at least one prismmember having a positive refracting power and a second unit having apositive refracting power, a primary image surface is formed between thefirst unit and the second unit, and the following condition (3) issatisfied:

0.1<P·PD·ZDD<5   (3)

[0030] where P is a pixel pitch (in μm) of the image display element, PDis a diameter (in mm) of the exit pupil, and ZDD is a distance long theoptical axis from the primary image surface to an optical element thatis located closest to the primary image surface.

[0031] An observation optical system that forms a primary image surfacein the path has a problem of optical performance degradation caused bydust or flaws on an optical element disposed in the vicinity of theprimary image surface where the beam of rays is relatively narrow.

[0032] If dust is caught on an optical element disposed in the vicinityof the primary image surface where the beam diameter is narrow, ablurred image of the dust is projected relatively large for observationand thus degrades the image quality.

[0033] In this case, under the condition where a dust particle with thesame size is caught on the surface, a coarser pixel pitch the imagedisplay element such as a LCD makes the image of the dust particle to beless conspicuous because resolution over the entire image is coarser,while a finer pixel pitch causes the blurred image of the dust particleto be more conspicuous because resolution of the image is finer. Also, alarger diameter of the exit pupil, or a larger value of NA causes alarger beam diameter at the first reflecting surface, and thus the dustparticle is projected relatively small to be less conspicuous. Also, alonger distance from the primary image surface to the optical elementdisposed closest to the primary image surface causes a larger beamdiameter at the first reflecting surface, and thus the dust particle isprojected relatively small to be less conspicuous.

[0034] Considering the discussions set forth above collectively, theapplicant has revealed that satisfaction of Condition (3) is desirablein the image display apparatus according to the fourth aspect.

[0035] If Condition (3) is satisfied, it is possible to achieve goodimage quality because dust, which would be caught on the reflectingsurface during fabrication, is inconspicuous as observed.

[0036] Failing to reach the lower limit value, 0.1 of Condition (3) isnot preferable, because it degrades image quality with dust caught onthe reflecting surface being conspicuous as a large image. Exceeding theupper limit value, 5 of Condition (3) is not preferable, because such aconfiguration is impractical with bulkiness of the optical system orwith degraded image quality over the entire field.

[0037] Also, an image display apparatus according to the fifth aspect ofthe present invention includes an image display element and anobservation optical system which forms an exit pupil for observation ofan image displayed on the image display element, wherein the observationoptical system includes a first unit having a positive refracting powerand a second unit, and the following condition (4) is satisfied:

0.02×10⁻² <α·P<2×10⁻²   (4)

[0038] where α is a field angle (in rad.) of the observation opticalsystem, and P is a pixel pitch (in μm) of the image display element.

[0039] In an observation optical system of an image display apparatusincluding a first unit having a positive refracting power and a secondunit, it is desirable that Condition (4) is satisfied for the purpose ofachieving high image quality while maintaining compact size of theapparatus. In an image display apparatus, a wider field angle canprovide more real ambience of the image, while an extremely wide fieldangle raises a problem of size increase of the observation opticalsystem. Also, a smaller pixel pitch is desirable in view of obtaining ahigher image quality, while an extremely small pixel pitch raises aproblem of increase in number of lenses required for compensation ofaberrations generated in the observation optical system, and accordinglyincrease in size of the entire image display apparatus.

[0040] Therefore, according to the fifth aspect, it is desirable tosatisfy Condition (4). Satisfaction of Condition (4) allows theapparatus to achieve high image quality with its size being maintainedcompact.

[0041] Failing to reach the lower limit value, 0.02×10⁻² of Condition(4) is not preferable, because it renders the field angle narrow or theobservation optical system large. Exceeding the upper limit value,2×10⁻² of Condition (4) is not preferable, because it prevents ahigh-definition image or renders the observation optical system large.

[0042] Also, according to the fifth aspect, it is more desirable thatthe following condition (5) is satisfied in addition to Condition (4):

0.05<P·LD<2   (5)

[0043] where LD is a distance (in mm) taken along the image centerbetween the last surface (a surface of the observation optical systemfarthest from the exit pupil along the path) of the observation opticalsystem and the exit pupil.

[0044] Satisfaction of Condition (5) allows the apparatus to be madecompact while good image quality being maintained.

[0045] Failing to reach the lower limit value, 0.05 of Condition (5) isnot preferable, because it requires a infeasible value of the distancebetween the last surface of the observation optical system and the exitpupil renders the observation optical system large.

[0046] Exceeding the upper limit value, 2 of Condition (5) is notpreferable, because it prevents a high-definition image or renders theobservation optical system large.

[0047] An image display apparatus according to the sixth aspect of thepresent invention includes an image display element, an observationoptical system which forms an exit pupil for observation of an imagedisplayed on the image display element, and a clip section, wherein theobservation optical system has a positive refracting power, and a framemember provided with said observation optical system is integrallyformed with the clip section.

[0048] Also, according to the first aspect, it is much desirable tosatisfy the following condition (6), further limiting Condition (1):

0.5<P·PD·ZD<2   (6)

[0049] Also, according to the second aspect, it is much desirable tosatisfy the following condition (7), further limiting Condition (2):

a<50   (7)

[0050] Also, according to the fourth aspect, it is much desirable tosatisfy the following condition (8), further limiting Condition (3):

0.5<P·PD·ZDD<2   (8)

[0051] Also, according to the fifth aspect, it is much desirable tosatisfy the following condition (9), further limiting Condition (4):

0.1×10⁻² <α·P<1×10⁻²   (9)

[0052] Also, according to the fifth aspect, it is much desirable tosatisfy the following condition (10), further limiting Condition (5):

0.1<P·LD<1   (10)

[0053] Also, according to the first aspect, it is preferable that theobservation optical system includes a first optical member having afirst surface that has an action of reflecting bundles of rays from theimage display element and a second optical member having an action offurther reflecting the bundles of rays reflected from the first surfaceand that a space between the first optical member and the second opticalmember is filled with gas.

[0054] Replacing a medium in a space between the first optical memberand the second optical member with gas can reduce the weight by thedifference.

[0055] Also, in the image display apparatus according to the firstaspect, it is preferable that at least one surface of the observationoptical system is composed of a diffraction element which is given alens function by diffraction effect.

[0056] Use of a diffraction element facilitates reduction in size andweight and allows chromatic aberration generated at the remainingsurfaces of the observation optical system to be cancelled.

[0057] Also, according to the first aspect, it is preferable that atleast one surface of the observation optical system has a curved surfaceshape to exert a power on bundles of rays, and that the curved surfaceshape is configured as a rotationally asymmetric surface shape tocompensate aberrations generated by decentering.

[0058] Also, according to the second aspect, it is preferable that atleast one another optical element disposed between the exit pupil andthe optical member is a diffraction element.

[0059] Also, according to the second aspect, it is preferred that theobservation optical system includes at least one prism member, that theprism member has an entrance surface via which bundles of rays emergentfrom the image display element enter the prism member, at least onereflecting surface which reflects the bundles of rays inside the prismmember, and an exit surface via which the bundles of rays exit out ofthe prism member, that the at least one reflecting surface has a curvedsurface shape to exert a power on bundles of rays, and that the curvedsurface shape is configured as a rotationally asymmetric surface shapeto compensate aberrations generated by decentering.

[0060] Also, according to the third aspect, it is preferable that atleast one surface in the observation optical system is configured as anoptical element that is given a lens function by diffraction effect.

[0061] Also, according to the third aspect, it is preferable that atleast one prism member in the observation optical system has an entrancesurface via which bundles of rays emergent from the image displayelement enter the prism, at least one reflecting surface which reflectsthe bundles of ryas inside the prism member, and an exit surface viawhich the bundles of rays exit out of the prism member, that the atleast one reflecting surface has a curved surface shape to exert a poweron the bundles of rays, and that the curved surface shape is configuredas a rotationally asymmetric surface shape to compensate aberrationsgenerated by decentering.

[0062] Also, according to the third aspect, it is preferable that thesecond unit has a positive refracting power.

[0063] Also, according to the third aspect, it is preferable that, foralignment of an optical axis of the image display element, an opticalaxis of the first unit and an optical axis of the second unit, the firstunit is adjusted so that, upon a first pinhole being arranged on an exitsurface side of the first unit and being aligned with the optical axisof the first unit and a second pinhole being arranged on an exit surfaceside of the second unit and being aligned with the optical axis of thesecond unit, and then upon a central portion of the image displayelement being made to flash as a point light source, the point lightsource is observable through the first pinhole and the second pinhole.

[0064] Also, according to the third aspect, the image displayed on theimage display element may be photographed by a photographing opticalsystem disposed on an exit pupil side.

[0065] Also, according to the third aspect, it is preferable that thefirst unit is fixed with adhesive after alignment of the optical axes.

[0066] Also, according to the fourth aspect, it is preferable that amember that has an action of interrupting stray light from the imagedisplay element is disposed between the image display element and theexit pupil.

[0067] Also, according to the fourth aspect, it is preferable that atleast one flare stop is disposed between the first unit and the secondunit.

[0068] Also, according to the fourth aspect, it is preferable that atleast one prism member of the observation optical system has an entrancesurface via which bundles of rays emergent from the image displayelement enter the prism member, at least one reflecting surface whichreflects the bundles of rays inside the prism member, and an exitsurface via which the bundles of rays exit out of the prism member, thatthe at least one reflecting surface has a curved surface shape to exerta power on bundles of rays, and that the curved surface shape isconfigured as a rotationally asymmetric surface shape to compensateaberrations generated by decentering.

[0069] Also, according to the fifth aspect, it is preferable that theobservation optical system includes a first unit having a positiverefracting power and a second unit.

[0070] In this configuration, it is preferable that the first unitincludes at least one prism member having a positive refracting power,that the prism member has an entrance surface via which bundles of raysemergent from the image display element enter the prism member, at leastone reflecting surface which reflects the bundles of rays inside theprism member, and an exit surface via which the bundles of rays exit outof the prism member, that the at least one reflecting surface has acurved surface shape to exert a power on bundles of rays, and that thecurved surface shape is configured as a rotationally asymmetric surfaceshape to compensate aberrations generated by decentering.

[0071] Also, in this configuration, it is preferable that the secondunit has a positive refracting power.

[0072] Also, according to the fifth aspect, it is preferable that atleast one surface in the observation optical system is an opticalelement that is given a lens function by diffraction effect.

[0073] Also, according to the sixth aspect, it is preferable that theobservation optical system includes a first unit having a positiverefracting power and a second unit.

[0074] In this configuration, it is preferable that the first unitincludes at least one prism member having a positive refracting power,that the prism member has an entrance surface via which bundles of raysemergent from the image display element enter the prism, at least onereflecting surface which reflects the bundles of rays inside the prismmember, and an exit surface via which the bundles of rays exit out ofthe prism, that the at least one reflecting surface has a curved surfaceshape to exert a power on bundles of rays, and that the curved surfaceshape is configured as a rotationally asymmetric surface shape tocompensate aberrations generated by decentering.

[0075] Also, in this configuration, it is preferable that the secondunit has a positive refracting power.

[0076] Also, according to the sixth aspect, it is preferable that atleast one surface in the observation optical system is a diffractionoptical element.

[0077] Also, a rotationally asymmetric surface used in the presentinvention may be configured as any one of an anamorphic surface, a toricsurface, and a free curved surface that defines only one plane ofsymmetry. Specifically, the surface is preferably configured as a freecurved surface that defines only one plane of symmetry.

[0078] Also, in the case where a prism member is used in the observationoptical system, a reflecting surface provided on the prism member may beconfigured as a plane-symmetric free curved surface defining only oneplane of symmetry.

[0079] This and other objects as well as features and advantages of thepresent invention will become apparent from the following detaileddescription of the preferred embodiments when taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080]FIG. 1 is a view to explain the principle of the image displayapparatus according to the present invention.

[0081]FIG. 2 is a sectional view of an essential portion of the imagedisplay apparatus according to the first embodiment of the presentinvention.

[0082]FIG. 3 is a sectional view of an essential portion of the imagedisplay apparatus according to the second embodiment of the presentinvention.

[0083]FIG. 4 is a sectional view of an essential portion of the imagedisplay apparatus according to the third embodiment of the presentinvention.

[0084]FIG. 5A is a sectional view of an essential portion of the imagedisplay apparatus of the reference example, and FIG. 5B is a sectionalview of an essential portion of the image display apparatus according tothe fourth embodiment of the present invention.

[0085]FIG. 6 is a sectional view of an essential portion of the imagedisplay apparatus according to the fifth embodiment of the presentinvention.

[0086]FIGS. 7A-7C show the image display apparatus according to thesixth embodiment of the present invention, where FIG. 7A is a sectionalview of an essential portion to schematically show configuration of theoptical members, FIG. 7B is a perspective views to show the outline ofthe image display apparatus, and FIG. 7C is an explanatory view to showthe situation where alignment of optical axes are made.

[0087]FIG. 8 is a sectional view of an essential portion of the imagedisplay apparatus according to the seventh embodiment of the presentinvention.

[0088]FIG. 9 is a sectional view of an essential portion of the imagedisplay apparatus according to the eighth embodiment of the presentinvention.

[0089]FIG. 10 is a sectional view of an essential portion of the imagedisplay apparatus according to the ninth embodiment of the presentinvention.

[0090]FIG. 11 is a sectional view of an essential portion of the imagedisplay apparatus according to the tenth embodiment of the presentinvention.

[0091]FIG. 12 is a sectional view of an essential portion of the imagedisplay apparatus according to the eleventh embodiment of the presentinvention.

[0092]FIG. 13 is a sectional view of an essential portion of the imagedisplay apparatus according to the twelfth embodiment of the presentinvention.

[0093]FIG. 14 shows an example of a prism applicable to the prism memberof the observation optical system according to the present invention.

[0094]FIG. 15 shows another example of a prism applicable to the prismmember of the observation optical system according to the presentinvention.

[0095]FIG. 16 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0096]FIG. 17 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0097]FIG. 18 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0098]FIG. 19 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0099]FIG. 20 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0100]FIG. 21 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0101]FIG. 22 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0102]FIG. 23 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0103]FIG. 24 shows still another example of a prism applicable to theprism member of the observation optical system according to the presentinvention.

[0104]FIG. 25 is a view of a head-mount type binocular image displayapparatus using the observation optical system according to the presentinvention, as it is fit to the head of an observer.

[0105]FIG. 26 is a sectional view of the image display apparatus shownin FIG. 25.

[0106]FIG. 27 is a view of a head-mount type monocular image displayapparatus using the observation optical system according to the presentinvention, as it is fit to the head of an observer.

[0107]FIG. 28 shows a desirable arrangement of the prism and thediffraction element in the first unit according to the presentinvention.

[0108]FIG. 29 shows a configuration example of an observation opticalsystem in an image display apparatus.

[0109]FIG. 30 shows another configuration example of an observationoptical system in an image display apparatus.

[0110]FIG. 31 is a view to show the principle of defining a diffractionelement (HOE) according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0111] Preceding the descriptions of the individual embodiments,detailed explanation is made of the structure and arrangement of opticalsurfaces used in the present invention.

[0112] In the image observation optical system according to the presentinvention, the axial chief ray is defined as a ray travelling from thecenter of the exit pupil through the center of the image displayelement. The optical axis, which is defined by the straight-line portionof the axial chief ray from the center of the exit pupil to the firstsurface of an optical member, is defined as Z axis. The axis thatintersects Z axis at right angles and that is parallel with a plane ofthe figure sheet is defined as Y axis. The axis that intersects Z axisat right angles and that intersects Y axis at right angles is defined asZ axis. The center of the exit pupil is determined as the origin of thecoordinate system for the observation optical system of the presentinvention. Also, according to the present invention, surface arrangementnumbers are assigned in order from the exit pupil through the imagedisplay element to conform to the reverse ray tracing. A direction ofthe axial chief ray from the exit pupil toward the image display elementis defined as a positive direction of Z axis. A direction of Y axis thatis toward the image display element is defined as a positive directionof Y axis. A direction of X axis that forms a right-hand system alongwith Y axis and Z axis is defined as a positive direction of Y axis.

[0113] Here, a free curved surface used in the present invention isdefined by the following equation (11) where Z axis appearing therein isthe axis of the free curved surface: $\begin{matrix}{Z = {{{cr}^{2}/\left\{ {1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}r^{2}}}} \right\}} + {\sum\limits_{j = 2}^{66}\quad {c_{j}X^{m}Y^{n}}}}} & (11)\end{matrix}$

[0114] The first term of Equation (11) expresses the spherical surfacecomponent. The second term of Equation (11) expresses the free curvedsurface component. In the term of the spherical surface component, crepresents a curvature at the vertex, k represents a conic constant, andr={square root}{square root over (X²+Y²)}.

[0115] The term of the free curved surface component is expanded asshown in the following equation (12): $\begin{matrix}\begin{matrix}{{\sum\limits_{j = 2}^{66}\quad {C_{j}X^{m}Y^{n}}} =} \\{{C_{2}X} + {C_{3}Y} +} \\{{C_{4}X^{2}} + {C_{5}{XY}} + {C_{6}Y^{2}} +} \\{{C_{7}X^{3}} + {C_{8}X^{2}Y} + {C_{9}{XY}^{2}} + {C_{10}Y^{3}} +} \\{{C_{11}X^{4}} + {C_{12}X^{3}Y} + {C_{13}X^{2}Y^{2}} + {C_{14}{XY}^{3}} + {C_{15}Y^{4}} +} \\{{C_{16}X^{5}} + {C_{17}X^{4}Y} + {C_{18}X^{3}Y^{2}} + {C_{19}X^{2}Y^{3}} + {C_{20}{XY}^{4}} + {C_{21}Y^{5}} +} \\{{C_{22}X^{6}} + {C_{23}X^{5}Y} + {C_{24}X^{4}Y^{2}} + {C_{25}X^{.}Y^{3}} + {C_{26}X^{2}Y^{4}} + {C_{27}{XY}^{5}} +} \\{{C_{28}Y^{6}} + {C_{29}X^{7}} + {C_{30}X^{6}Y} + {C_{31}X^{5}Y^{2}} + {C_{32}X^{4}Y^{3}} + {C_{33}X^{3}Y^{4}} +} \\{{C_{34}X^{2}Y^{5}} + {C_{35}{XY}^{6}} + {C_{36}Y^{7}\quad \ldots}}\end{matrix} & (12)\end{matrix}$

[0116] where C_(j) (j is integer equal to or greater than 2) is acoefficient.

[0117] In general, a free curved surface as expressed above does nothave a plane of symmetry along X-Z plane or along Y-Z plane. However,according to the present invention, upon all terms with odd-numberedpowers of X being nullified, the free curved surface can define only oneplane of symmetry that is parallel to Y-Z plane. Such a free curvedsurface is obtained, for example, by setting values of the coefficientsC₂, C₅, C₇, C₉, C₁₂, C₁₄, C₁₆, C₁₈, C₂₀, C₂₃, C₂₅, C₂₇, C₂₉, C₃₁, C₃₃,C₃₅ . . . of the terms in Equation (11) at zero.

[0118] Alternatively, upon all terms with odd-numbered powers of Y beingnullified, the free curved surface can define only one plane of symmetrythat is parallel to X-Z plane. Such a free curved surface is obtained,for example, by setting values of the coefficients C₃, C₅, C₈, C₁₀, C₁₂,C₁₄, C₁₇, C₁₉, C₂₁, C₂₃, C₂₅, C₂₇, C₃₀, C₃₂, C₃₄, C₃₆ . . . of the termsin Equation (11) at zero.

[0119] Also, a free curved surface that defines one of theabove-mentioned planes of symmetry is arranged so that its plane ofsymmetry corresponds to the decentering direction of the optical system.That is, a free curved surface defining a plane of symmetry parallel toY-Z plane is combined with an optical system having decenteringdirection along Y axis, and a free curved surface defining a plane ofsymmetry parallel to X-Z plane is combined with an optical system havingdecentering direction along X axis, to effectively compensaterotationally asymmetric aberrations caused by decentering and to improvefacility for fabrication.

[0120] Equation (11) is presented as one example that can define a freecurved surface. Even if the free curved surface having only one plane ofsymmetry according to the present invention is defined by any differentexpression, it is still effective in compensation of rotationallyasymmetric aberrations caused by decentering and in improvement offacility for fabrication, as a matter of course.

[0121] For instance, the free curved surface can be defined by Zernikepolynomial, also. The configuration of the surface is defined by thefollowing equations (13). Z axis appearing in Equation (13) representsthe axis of Zernike polynomial. The rotationally asymmetric surface isdefined by height in Z axis, in terms of polar coordinate, in referenceto X—Y plane. $\begin{matrix}\begin{matrix}{X = {R \times {\cos (A)}}} \\{Y = {R \times {\sin (A)}}} \\\begin{matrix}{Z = {D_{2} +}} \\{{{D_{3}R\quad {\cos (A)}} + {D_{4}R\quad {\sin (A)}} +}} \\{{{D_{5}R^{2}\quad {\cos \left( {2A} \right)}} + {D_{6}\left( {R^{2} - 1} \right)} + {D_{7}R^{2}\quad {\sin \left( {2A} \right)}} +}} \\{{{D_{8}R^{3}\quad {\cos \left( {3A} \right)}} + {{D_{9}\left( {{3R^{3}} - {2R}} \right)}{\cos (A)}} + {{D_{10}\left( {{3R^{3}} - {2R}} \right)}{\sin (A)}} +}} \\{{{D_{11}R^{3}{\sin \left( {3A} \right)}} + {D_{12}R^{4}{\cos \left( {4A} \right)}} + {{D_{13}\left( {{4R^{4}} - {3R^{2}}} \right)}{\cos \left( {2A} \right)}} +}} \\{{{D_{14}\left( {{6R^{4}} - {6R^{2}} + 1} \right)} + {{D_{15}\left( {{4R^{4}} - {3R^{2}}} \right)}{\sin \left( {2A} \right)}} + {D_{16}R^{4}{\sin \left( {4A} \right)}} +}} \\{{{D_{17}R^{5}{\cos \left( {5A} \right)}} + {{D_{18}\left( {{5R^{5}} - {4R^{3}}} \right)}{\cos \left( {3A} \right)}} +}} \\{{{{D_{19}\left( {{10R^{5}} - {12R^{3}} + {3R}} \right)}{\cos (A)}} +}} \\{{{{D_{20}\left( {{10R^{5}} - {12R^{3}} + {3R}} \right)}{\sin (A)}} + {{D_{21}\left( {{5R^{5}} - {4R^{3}}} \right)}{\sin \left( {3A} \right)}} +}} \\{{{D_{22}R^{5}{\sin \left( {5A} \right)}} + {D_{23}R^{6}{\cos \left( {6A} \right)}} + {{D_{24}\left( {{6R^{6}} - {5R^{4}}} \right)}{\cos \left( {4A} \right)}} +}} \\{{{{D_{25}\left( {{15R^{6}} - {20R^{4}} + {6R^{2}}} \right)}{\cos \left( {2A} \right)}} +}} \\{{{D_{26}\left( {{20R^{6}} - {30R^{4}} + {12R^{2}} - 1} \right)} +}} \\{{{{D_{27}\left( {{15R^{6}} - {20R^{4}} + {6R^{2}}} \right)}{\sin \left( {2A} \right)}} +}} \\{{{{D_{28}\left( {{6R^{6}} - {5R^{4}}} \right)}{\sin \left( {4A} \right)}} + {D_{29}R^{6}{\sin \left( {6A} \right)}\quad \ldots}}}\end{matrix}\end{matrix} & (13)\end{matrix}$

[0122] where R is a distance from Z axis in X—Y plane, A is an azimuthabout Z axis expressed by a rotation angle from Z axis, and D_(m) (m isinteger equal to or greater than 2) is a coefficient. It is noted thatEquation (13) corresponds to a free curved surface that is symmetric inX direction.

[0123] Configuration of an anamorphic surface is defined by thefollowing equation (14). The normal to the optical surface at the originof the surface shape is defined as the axis of the anamorphic surface.

Z=(C _(x) ·X ² +C _(y) ·Y ²)/[1+{1−(1+K _(x))C _(x) ² {square root}X²−(1+K _(y))C _(y) ² ·Y ²}^(1/2) ]+ΣR _(n){(1−P _(n))X ²+(1+P _(n))Y²}^((n+1))   (14)

[0124] Here, if it is assumed that n is from 1 to 4 (polynomial ofdegree 4), for example, Equation (14) is expanded as the followingexpression (15): $\begin{matrix}\begin{matrix}{Z = {\left( {{C_{x} \cdot X^{2}} + {C_{y} \cdot Y^{2}}} \right)/\left\lbrack {1 + \left\{ {1 - {\left( {1 + K_{x}} \right){C_{x}^{2} \cdot X^{2}}} -} \right.} \right.}} \\{\left. \left. \quad {\left( {1 + K_{y}} \right){C_{y}^{2} \cdot Y^{2}}} \right\}^{1/2} \right\rbrack +} \\{\quad {{R_{1}\left\{ {{\left( {1 - P_{1}} \right)X^{2}} + {\left( {1 + P_{1}} \right)Y^{2}}} \right\}^{2}} +}} \\{\quad {{R_{2}\left\{ {{\left( {1 - P_{2}} \right)X^{2}} + {\left( {1 + P_{2}} \right)Y^{2}}} \right\}^{3}} +}} \\{\quad {{R_{3}\left\{ {{\left( {1 - P_{3}} \right)X^{2}} + {\left( {1 + P_{3}} \right)Y^{2}}} \right\}^{4}} +}} \\{\quad {R_{4}\left\{ {{\left( {1 - P_{4}} \right)X^{2}} + {\left( {1 + P_{4}} \right)Y^{2}}} \right\}^{5}}}\end{matrix} & (15)\end{matrix}$

[0125] where Z is an amount of deviation from a plane tangent to theorigin of the surface shape, C_(x) is a curvature in X-axis direction,C_(y) is a curvature in Y-axis direction, K_(x) is a conical coefficientin X-axis direction, K_(y) is a conical coefficient in Y-axis direction,R_(n) is a rotationally symmetric component of a spherical surface term,and P_(n) is a rotationally asymmetric component of an asphericalsurface term. A radius of curvature R_(x) in X-axis direction and aradius of curvature R_(y) in Y-axis direction are correlated with thecurvatures C_(x), and C_(y), respectively, as follows:

R _(x)=1/C _(x) , R _(y)=1/C _(y).

[0126] Regarding the toric surface, there are two kinds; i.e. X toricsurface and Y toric surface, which are expressed by the followingequations (16), (17), respectively. The normal to the optical surface atthe origin of the surface shape is defined as the axis of the toricsurface.

[0127] X toric surface is defined as follows:

F(X)=C _(x) ·X ²/[1+{1−(1+K)C_(x) ² ·X ²}^(1/2) ]+AX ⁴ +BX ⁶ +CX ⁸ +DX¹⁰ . . . Z=F(X)+(½)C _(y) {Y ² +Z ² −F(X)^(2 })  (16)

[0128] Y toric surface is defined as follows:

F(Y)=C _(y) ·Y ²/[1+{1−(1+K)C _(y) ² ·Y ²}^(1/2) ]+AY ⁴ +By ⁶ +CY ⁸ +DY¹⁰ . . . Z=F(Y)+(½)C _(x) {X ² +Z ² −F(Y)²}  (17)

[0129] where Z is an amount of deviation from a plane tangent to theorigin of the surface shape, C_(x) is a curvature in X-axis direction,C_(y) is a curvature in Y-axis direction, K is a conical coefficient,and A, B, C, and D are aspherical coefficients. A radius of curvatureR_(x) in X-axis direction and a radius of curvature R_(y) in Y-axisdirection are correlated with the curvatures C_(x), and C_(y),respectively, as follows:

R_(x)=1/C _(x) , R _(y)=1/C _(y).

[0130] Configuration of a rotationally symmetric aspherical surface isdefined by the following equation (18). Z axis appearing in Equation(18) represents the axis of the rotationally symmetric asphericalsurface.

Z=(Y ² /R)/[1+{1−P(Y ² /R ²)}^(1/2) ]+A ₄ Y ⁴ +A ₆ Y ⁶ +A ₈ Y ⁸ +A ₁₀ Y¹⁰   (18)

[0131] where Y is a direction perpendicular to Z, R is a radius ofparaxial curvature, P is a conical coefficient, and A₄, A₆, A₈, A₁₀ areaspherical coefficients.

[0132] The diffraction element (HOE) used in the present invention isdefined as follows. FIG. 31 is a view to show the principle of definingHOE according to the present invention.

[0133] Ray tracing for a ray with wavelength λ incident at and emergentfrom any point P on the HOE surface is given by the following equation(19), which uses the optical path difference function Φ₀ defined for areference wavelength λ₀=HWL on the HOE surface:

n _(d) Q _(d) ·N=n _(i) Q _(i) ·N+m(λ/λ₀)∇Φ₀ ·N   (19)

[0134] where N is a vector of the normal to the HOE surface, n_(i)(n_(d)) is a refractive index on the incident side (emergent side),Q_(i) (Q_(d)) is a vector (unit vector) of incidence (emergence), andm=HOR is a diffraction order of emergent light.

[0135] If the HOE is fabricated (defined) by two point light sourceswith the reference wavelength λ₀, specifically by interference betweenobject rays emanating from the point P₁=(HY1, HY2, HY3) and referencerays emanating from the point P₂=(HX2, HY2, HZ2) as shown in FIG. 31,the following equation is satisfied: $\begin{matrix}{\Phi_{0} = \Phi_{0}^{2P}} \\{= {{n_{2} \cdot s_{2} \cdot r_{2}} - {n_{1} \cdot s_{1} \cdot r_{1}}}}\end{matrix}$

[0136] where r₁ (r₂) is a distance (>0) from the point P₁ (P₂) to apredetermined coordinate point on the HOE, n₁ (n₂) is a refractive indexof the point P₁ (P₂)-side medium by which the HOE was arranged duringfabrication (definition), s₁=HV1, and s₂=HV2 are signs to take intoconsideration the travelling direction of light. In the case where thelight source is a divergent light source (real point light source), thesign is set to be REA=+1, while in the case where the light source is aconvergent light source (virtual point light source), the sign is set tobe VIR=−1. It is noted that in defining a HOE in lens data, therefractive index n₁ (n₂) of the medium in which the HOE was arrangedduring fabrication is the refractive index of the medium that isadjacent to the HOE on the side of the point P₁ (P₂).

[0137] In general cases, reference rays and object rays used tofabricate a HOE are not limited to spherical waves. In these cases, theoptical path difference function Φ₀ of HOE can be defined by thefollowing equation (20) in which an additional phase term Φ₀ ^(Poly)(optical path difference function for the reference wavelength λ₀)expressed by polynomial terms is added:

Φ₀=Φ₀ ^(2P)+Φ₀ ^(Poly)   (20)

[0138] The polynomial Φ₀ ^(Poly) is given by: $\begin{matrix}{\Phi_{0}^{Poly} = {\sum\limits_{j}^{\quad}\quad {H_{j} \cdot x^{m} \cdot y^{n}}}} \\{= {{H_{1}x} + {H_{2}y} + {H_{3}x^{2}} + {H_{4}{xy}} + {H_{5}y^{2}} +}} \\{{{H_{6}x^{3}} + {H_{7}x^{2}y} + {H_{8}{xy}^{2}} + {H_{9}y^{3}} + \cdots}}\end{matrix}$

[0139] and can be defined, in general, by:

j={(m+n)² +m+3n}/2

[0140] where H_(j) is the coefficient of each term.

[0141] Furthermore, for convenience in optical designing, the opticalpath difference function Φ₀ may be expressed only by the additional termas follows:

Φ₀=Φ₀ ^(Poly)

[0142] whereby the HOE can be defined. For example, if the two pointlight sources P₁ and P₂ coincide, the component Φ₀ ^(2P) of the opticalpath difference function Φ₀ derived from interference becomes zero. Thiscondition corresponds to the case where the optical path differencefunction is expressed only by the additional terms (polynomialexpression).

[0143] The above descriptions regarding HOE are made in reference to alocal coordinate system determined by the HOE origin (O in FIG. 31).

[0144] An example of the parameter set to define the HOE is shown below:Surface Radius of Arrangement No. Curvature Separation object surface ∞∞ stop ∞ 100 2 150 −75

[0145] HOE:

HV 1 (s ₁)=REA (+1)

HV 2 (s ₂)=VIR (−1)

HOR (m)=1

HX 1=0, HY 1=−3.40×10⁹ , HZ 1=−3.40×10⁹

HX 2=0, HY 2=2.50×10, HZ 2=−7.04×10

HWL (λ₀)=544

H ₁=−1.39×10⁻²¹ H ₂=−8.57×10⁻⁵ H ₃=−1.50×10⁻⁴

[0146] In reference to the drawings, the individual embodiments aredescribed below.

[0147] In each embodiment, as shown in FIG. 2, the axial chief ray isdefined as a ray travelling from the center of an exit pupil 4 (therotation center of an eyeball of an observer) to the center of an imagedisplay element 1 such as a LCD via a first optical member 2 and asecond optical member 3. The optical axis, which is defined by thestraight line portion of the axial chief ray until it intersects theentrance surface of the first optical member 2, is defined as Z axis.The axis that intersects Z axis at right angles and that is parallelwith a plane of the figure sheet is defined as Y axis. The axis thatintersects the optical axis and Y axis at right angles is defined as Xaxis. The center of the exit pupil 4 is determined as the origin of thiscoordinate system. The direction of the axial chief ray from the exitpupil 4 toward the image display element 1 is defined as a positivedirection of Y axis. A direction of Y axis that is toward the imagedisplay element 1 is defined as a positive direction of Y axis. Adirection of X axis that forms a right-hand system along with Y axis andZ axis is defined as a positive direction of X axis.

[0148] In the case where a prism is applied to an embodiment of thepresent invention, the prism is decentered in Y-Z plane. Also, eachrotationally asymmetric free curved surface provided for the prism hasthe only one plane of symmetry on Y-Z plane.

[0149] For each decentered surface, amount of displacement (expressed byX, Y, Z for components in X-axis direction, Y-axis direction, Z-axisdirection, respectively) of the vertex position of the surface from theorigin of the corresponding coordinate system and tilt angles (α, β, γ(°)) of the center axis (=Z axis in Equation (11) for a free curvedsurface) of the surface in reference to X axis, Y axis and Z axis,respectively, are given. A positive value of α or β meanscounterclockwise rotation in reference to the positive direction of thecorresponding axis, while a positive value of γ means clockwise rotationin reference to the positive direction of Z axis. Other parameters suchas radius of curvature of spherical surface, surface separation,refractive index of medium, and Abbe's number are given by theconventional method.

[0150] Shape of the free curve surface used in the present invention isdefined by Equation (11), where Z axis corresponds to the axis of thefree curved surface. However, even if Equation (13) is applied, it doesnot affect the function and effect of the invention, as a matter ofcourse.

First Embodiment

[0151] In reference to FIG. 2, an image display apparatus according tothe first embodiment of the present invention is described. In FIG. 2,the reference numeral 1 represents an image display element such as aLCD, the reference numeral 2 represents a thin platelike first opticalmember, the reference numeral 2 ₁ represents a reflecting surface of thefirst optical member 2, the reference numeral 3 represents a thinplatelike second optical member, the reference numeral 3 ₁ represents areflecting surface of the second optical member 3, the reference numeral4 represents an exit pupil, and the reference numeral 5 represents aflare stop disposed in front of the image display element 1.

[0152] According to this embodiment, a non-coaxial (decentered)observation optical system having a positive refracting power iscomposed optical members 2 and 3. In the observation optical system,bundles of rays emergent from the image display element 1 are reflectedat the reflecting surface 2 ₁ of the optical member 2, to be directedtoward the optical member 3, and the bundles of rays reflected at thereflecting surface 3 ₁ of the optical member 3 are re-directed towardthe optical member 2, to be transmitted therethrough.

[0153] The reflecting surface 3 ₁ of the optical member 3 is shaped as acurved surface which exerts a power on bundles of rays. The curvedsurface is a rotationally asymmetric surface constructed and arranged tocompensate aberrations generated by decentering. Whereby, aberrationsare compensated in good condition while compact-sizing of the apparatusis achieved.

[0154] Also, the first optical member 2 is configured as a Lippmannvolume hologram, and the second optical member 3 is configured as a freecurved surface lens.

[0155] A space between the first optical member 2 and the second opticalmember is not filled with glass or plastic, but with gas, to therebyachieve weight reduction of the observation optical system.

[0156] According to this embodiment, a diffraction element and a freecurved surface (free curved surface lens) are used as the reflectingsurfaces of the observation optical system. However, not limited tothese, an aspherical surface, a toric surface, a polarizing plate or thelike may be used as a reflecting surface.

[0157] Also, according to this embodiment, the pixel pitch P of theimage display element 1 is 12 μm, the distance ZD is 15 mm, and thepupil diameter PD is 4.5 mm.

Second Embodiment

[0158] In reference to FIG. 3, an image display apparatus according tothe second embodiment of the present invention is described.

[0159] According to this embodiment, a non-coaxial (decentered)observation optical system having a positive refracting power iscomposed of optical members 2 and 3. In the observation optical system,bundles of rays emergent from the image display element 1 are reflectedat the reflecting surface 2 ₁ of the optical member 2, to be directedtoward the optical member 3, and the bundles of rays reflected at thereflecting surface 3 ₁ of the optical member 3 are re-directed towardthe optical member 2, to be transmitted therethrough.

[0160] The reflecting surface 3 ₁ of the optical member 3 is shaped as acurved surface which exerts a power on bundles of rays. The curvedsurface is a rotationally asymmetric surface constructed and arranged tocompensate aberrations generated by decentering. Whereby, aberrationsare compensated in good condition while compact-sizing of the apparatusis achieved. Also, this embodiment is different from the embodiment ofFIG. 2 in that the optical member 2 and the optical member 3 areintegrally formed. However, the optical member 2 and the optical member3 maybe formed separately.

[0161] Also, according to this embodiment, the first optical member 2 isconfigured as a Lippmann volume hologram, and the second optical member3 is configured as a free curved surface lens.

[0162] A space between the first optical member 2 and the second opticalmember 3 is not filled with glass or plastic, but with gas, to therebyachieve weight reduction. The mouth of the space may be closed up with atransparent optical member for the purpose of preventing dust to beentrapped in the space filled with gas.

[0163] According to this embodiment also, a diffraction element and afree curved surface (free curved surface lens) are used as thereflecting surfaces of the observation optical system, as in theembodiment of FIG. 2. However, not limited to these, an asphericalsurface, a toric surface, a polarizing plate or the like may be used asa reflecting surface.

[0164] Also, according to this embodiment, the pixel pitch P of theimage display element 1 is 10 μm, the distance ZD is 20 mm, and thepupil diameter PD is 4.5 mm.

Third Embodiment

[0165] In reference to FIG. 4, an image display apparatus according tothe third embodiment of the present invention is described.

[0166] According to this embodiment, a non-coaxial (decentered)observation optical system having a positive refracting power iscomposed of optical members 2 and 3. In the observation optical system,bundles of rays emergent from the image display element 1 are reflectedat the reflecting surface 2 ₁ of the optical member 2, to be directedtoward the optical member 3, are reflected at the reflecting surface 3 ₁of the optical member 3, to be re-directed toward the optical member 2,and are reflected at the reflecting surface 2 ₁ of the optical member 2,to be re-directed toward the optical member 3, and then are transmittedthrough the optical member 3.

[0167] The optical member 2 is shaped as a curved surface which exerts apower on bundles of rays. The curved surface is a rotationallyasymmetric surface constructed and arranged to compensate aberrationsgenerated by decentering. Whereby, aberrations are compensated in goodcondition while compact-sizing of the apparatus is achieved.

[0168] Also, the optical member 2 and the optical member 3 areintegrally formed. However, the optical member 2 and the optical member3 may be formed separately.

[0169] Also, the first optical member 2 is configured as a Lippmannvolume hologram, and the second optical member 3 is configured as a freecurved surface lens.

[0170] A space between the first optical member 2 and the second opticalmember 3 is not filled with glass or plastic, but with gas, to therebyachieve weight reduction. The mouth of the space may be closed up with atransparent optical member for the purpose of preventing dust to beentrapped in the space filled with gas.

[0171] According to this embodiment also, a diffraction element and afree curved surface (free curved surface lens) are used as thereflecting surfaces of the observation optical system, as in theembodiment of FIG. 2. However, not limited to these, an asphericalsurface, a toric surface, a polarizing plate or the like may be used asa reflecting surface.

[0172] Also, according to this embodiment, the pixel pitch P of theimage display element 1 is 8 μm, the distance ZD is 10 mm, and the pupildiameter PD is 3.5 mm.

Fourth Embodiment

[0173] In reference to FIGS. 5A-5B, an example in which flare byundesired order rays associated with use of a HOE is reduced ispresented as the fourth embodiment. According to the reference exampleshown in FIG. 5A, a volume hologram (HOE) is used for a thin platelikesecond optical member 3, to generate flare as a bundle 7 of rays from anemitting point 6 such as the sun or an electric lamp is diffracted atthe HOE 3. On the other hand, according to the present invention shownin FIG. 5B, the bundle 7 of rays from the emitting point 6 isinterrupted to a practically harmless level, to suppress generation offlare. Specifically, arranging an optical member 8 so that thediffraction element 3 is positioned between the exit pupil 4 and theoptical member 8 prevents flare from being generated.

Fifth Embodiment

[0174] In reference to FIG. 6, an image display apparatus according tothe fifth embodiment of the present invention is described. The fifthembodiment is a modified example of the fourth embodiment. The imagedisplay apparatus according to this embodiment is provided with a prismmember 9 in place of the first optical member 2 and the second opticalmember 3 of the fourth embodiment. The prism member 9 has a firstreflecting surface 9 ₁ and a second reflecting surface 9 ₂. The secondreflecting surface 9 ₂ of the prism member 9 is composed of a volumehologram (HOE). The optical member 8 satisfying Condition (2) suppressesgeneration of flare, which should have been caused by the bundle 7 ofrays from the emitting point 6 diffracted by the HOE and introduced intothe eye, by interrupting the bundle 7 of rays from the emitting point 6to a practically harmless level. Specifically, arranging the opticalmember 8 so that the diffraction element 3 is positioned between theexit pupil 4 and the optical member 8 prevents flare from beinggenerated.

Sixth Embodiment

[0175] In reference to FIGS. 7A-7C, an image display apparatus accordingto the sixth embodiment of the present invention is described. In thedrawings, the reference numeral 1 represents an image display elementsuch as a LCD, the reference numeral 2 represents a first unit of theobservation optical system composed of a single prism member having apositive refracting power, the reference numeral 3 represents a secondunit of the observation optical system composed of a volume hologram(HOE) having a positive refracting power, and the reference numeral 4represents an exit pupil. Also, the reference symbols P′, P″ are firstand second pinholes, respectively, used in optical axis alignment. Thereference numeral 10 represents a frame member which holds the firstunit of the observation optical system, the reference numeral 11 is aframe member which holds the second unit of the observation opticalsystem. The frame member 10 and the frame member 11 are supported by aframe member 12 having an oblong shape. The frame member 10 isconstructed and arranged to be movable in reference to the frame member12 so that optical axis alignment of the first unit and the second unitcan be performed by moving the frame member 10.

[0176] In alignment process of the optical axes of the image displayelement 1, the first unit and the second unit, the pinhole P′ isarranged on the exit surface side of the first unit and is aligned withthe optical axis of the first unit, the pinhole P″ is arranged on theexit surface side of the second unit and is aligned with the opticalaxis of the second unit, and a central portion of the image displayelement 1 is made to flash as a point light source. Then, optical axisalignment can be achieved by adjusting the first unit so that the pointlight source is observable through the first pinhole and the secondpinhole.

[0177] After optical axis alignment, it is desirable to fix the framemember 10 with adhesive.

[0178] Also, according to this embodiment, we can perform the opticalaxis alignment while monitoring an image on a LCD upon arranging aphotographing optical system including an image sensor 14 on the exitpupil side.

Seventh Embodiment

[0179] In reference to FIG. 8, an image display apparatus according tothe seventh embodiment of the present invention is described. Accordingto this embodiment, a flare stop 5 is disposed between a first unit 2composed of a prism member having a positive refracting power and asecond unit 3 composed of a prism member having a positive refractingpower so as to prevent flare. Also, in the observation optical system ofthis embodiment, compact-sizing of the apparatus can be achieved by thearrangement where an image is once formed at the position indicated bythe reference numeral 15. The symbol ZDD shown in the drawing representsa distance from the primary image surface to the second unit, whichsatisfies Condition (3), to prevent flare or the like caused by dust.Also, a member 16 having a function of interrupting stray light from theimage display element 1 is disposed between the image display element 1and the exit pupil 4. A frame member 12 holds the first unit 2, and aframe member 11 holds the second unit 3.

[0180] The prism member 2 constituting the first unit has an entrancesurface via which bundles of rays emergent from the image displayelement 1 enter the prism member, at least one reflecting surface whichreflect the bundles of rays inside the prism member, and an exit surfacevia which the bundles of rays exit out of the prism member. The at leastone reflecting surface has a curved surface shape to exert a power onbundles of rays. The curved surface shape is configured as arotationally asymmetric surface shape to compensate aberrationsgenerated by decentering.

Eighth Embodiment

[0181] In reference to FIG. 9, an example that achieves a compact imagedisplay apparatus is presented as the eighth embodiment. According tothis embodiment, a first unit 2 having a positive refracting power and asecond unit 3 constitutes a non-coaxial (decentered) observation opticalsystem having a positive refracting power. The first unit 2 is composedof a prism member, which has an entrance surface via which bundles ofrays emergent from the image display element enter the prism member, atleast one reflecting surface which reflect the bundles of rays insidethe prism member, and an exit surface via which the bundles of rays exitout of the prism member. The at least one reflecting surface has acurved surface shape to exert a power on bundles of rays. The curvedsurface shape is configured as a rotationally asymmetric surface shapeto compensate aberrations generated by decentering. The first unit 2 isconfigured so that reflection takes place three times inside the prismmember for the purpose of achieving compact-sizing. The second unit 3 iscomposed of a HOE and has an action of canceling chromatic aberrationgenerated in the first unit 2.

[0182] Also, a first frame 17 holds the first unit 2 and a second frame18 holds the second unit 3. The first frame 17 and the second frame 18are connected at a part thereof to form an integral member. Also, theHOE 3 is arranged to be substantially perpendicular to a ray along theoptical axis emergent from the HOE to enter the pupil. Therefore, it ispossible to construct the second frame 18 to be thin.

[0183] Also, in FIG. 9, the reference numeral 19 represents a controlsection for outputting video to the image display element, the referencenumeral 20 represents a recording section of the image to be output tothe image display element, the reference numeral 21 represents wiringwhich transmits data from the recording section 20 to the controlsection 19. It is noted that data communication may be performedwirelessly without using the wiring 21.

[0184] Also, in FIG. 9, the reference numeral 22 represents a clip usedto attach the image display apparatus of the present invention tospectacles or sunglasses, the reference numeral 23 represents a frame ofspectacles, and the reference numeral 24 represents a lens section ofthe spectacles. The clip 22 connects with the first frame 17 so thatclipping the frame 23 of spectacles with the clip 22 allows thespectacles to support the first frame member 17.

[0185] Also, in FIG. 9, the reference numeral 25 represents a window totake in external light for the purpose of illuminating the image displayelement such as a LCD. If the external light is weak, an illuminatingmeans (not shown) built in the frame 17 is used. On the other hand, ifthe apparatus is used in a site where external light is intense such asoutdoors, illumination of the image display element such as a LCD can beperformed with external light via the window 25 without using theilluminating means. The window 25 may be constructed of a lens or a HOEelement for the purpose of taking in the external light efficiently.

[0186] Also, in FIG. 9, the reference numeral 26 represents an axialchief ray travelling along the image center, and the reference numeral27 represents an off-axial chief ray. According to this embodiment,satisfaction of Condition (4) achieves high image quality of theobservation optical system, while satisfaction of Condition (5) achievescompact-sizing of the observation optical system.

[0187] Also, the recording section 20 may be configured as a cellularphone or a portable intelligence terminal. Electric power for drivingthe LCD or for illumination may be supplied from the recording section20.

Ninth Embodiment

[0188] In reference to FIG. 10, an image display apparatus according tothe ninth embodiment of the present invention is described. The ninthembodiment is a modification example of the eighth embodiment. The ninthembodiment differs from the eighth embodiment in the configuration ofthe observation optical system. Specifically, the HOE 3 is arranged notto be substantially perpendicular to a ray along the optical axisemergent from the HOE to enter the pupil. The HOE 3 is directed so thatthe incident angle β of this axial ray is substantially equal to theemergent angle β′ thereof, to weaken the power of the HOE. Therefore,according to this embodiment, this arrangement has an effect ofsuppressing flare by undesired order rays generated at the HOE.

[0189] In particular, according to this embodiment, satisfaction of thefollowing condition (21) makes it possible to suppress generation offlare by undesired order rays and thus to obtain a good imagingperformance:

0.5<|β/β′|<1.5   (21)

[0190] In addition, it is much desirable to satisfy the followingcondition (22), further limiting Condition (21):

0.8<|β/β′|<1.2   (22)

Tenth Embodiment

[0191] In reference to FIG. 11, an image display apparatus according tothe tenth embodiment of the present invention is described. The tenthembodiment is another modification example of the eighth embodiment.This embodiment differs from the eighth embodiment in the configurationof the observation optical system. Specifically, it is characterized byuse of a reflecting surface 3 ₁ having a positive refracting power asthe second unit 3. It is given the effect of precluding flare caused byundesired order rays generated at a HOE element by absence of the HOEelement, not like the eighth embodiment. Also, configuring thereflecting surface 3 ₁ of the second unit 3 as a free curved surfacemakes it possible to compensate aberrations caused by decentering ingood condition.

Eleventh Embodiment

[0192] In reference to FIG. 12, an image display apparatus according tothe eleventh embodiment of the present invention is described. Theeleventh embodiment is a modification example of the tenth embodiment.The eleventh embodiment differs from the tenth embodiment in theconfiguration of the observation optical system. Specifically, it ischaracterized by use of a two-reflection type prism optical system inthe first unit 2.

Twelfth Embodiment

[0193] In reference to FIG. 13, an image display apparatus according tothe twelfth embodiment of the present invention is described. Thetwelfth embodiment is another modification example of the tenthembodiment. The twelfth embodiment differs from the tenth embodiment inthe configuration of the observation optical system. The observationoptical system composed of a reflecting surface 28 alone achieves weightreduction.

[0194] Also, the prism used in the observation optical system accordingto the present invention is not limited to the types of theabove-described embodiments. For example, the prism shown in FIGS. 14-24may be used.

[0195] In the case of FIG. 14, a prism P is provided with a firstsurface 32, a second surface 33, and a third surface 34. The firstsurface 32, the second surface 33, and the third surface 34 areconstructed and arranged as an exit surface, a reflecting surface, andan entrance surface, respectively. The prism P is configured so thatlight from a LCD 36 enters the prism as being refracted at the thirdsurface 34 thereof, is reflected at the second prism 33, exits out ofthe prism as being refracted at the first surface 32 thereof, and thenis imaged on an image surface 31.

[0196] In the case of FIG. 15, a prism P is provided with a firstsurface 32, a second surface 33, and a third surface 34. The firstsurface 32 is constructed and arranged to act both as a first reflectingsurface and an exit surface. The second surface 33 is constructed andarranged to act both as a third reflecting surface and an entrancesurface. The third surface 34 is constructed and arranged as a secondreflecting surface. The prism P is configured so that light from a LCD36 enters the prism as being refracted at the second surface 33 thereof,is reflected at the first surface 32, and is reflected at the thirdsurface 34, then is reflected at the second surface 33, exits out of theprism as being refracted at the first surface 32 thereof, and is imagedon an image surface 31.

[0197] In the case of FIG. 16, a prism P is provided with a firstsurface 32, a second surface 33, a third surface 34, and a fourthsurface 35. The first surface 32 is constructed and arranged as an exitsurface. The second surface 33 is constructed and arranged as a thirdreflecting surface. The third surface 34 is constructed and arranged toact both as an entrance surface and a second reflecting surface. Thefourth surface 35 is constructed and arranged as a first reflectingsurface. The prism P is configured so that light from a LCD 36 entersthe prism as being refracted at the third surface 34 thereof, isreflected at the fourth surface 35, then is reflected at the thirdsurface 34, is reflected at the second surface 33, exits out of theprism as being refracted at the first surface 32 thereof, and is imagedon an image surface 31.

[0198] In the case of FIG. 17, a prism P is provided with a firstsurface 32, a second surface 33, a third surface 34, and a fourthsurface 35. The first surface 32 is constructed and arranged as an exitsurface. The second surface 33 is constructed and arranged to provide,at different positions on the very same surface, a region acting as afirst reflecting surface and a region acting as a third reflectingsurface. The third surface 34 is constructed and arranged as a secondreflecting surface. The fourth surface 35 is constructed and arranged asan entrance surface. The prism P is configured so that light from a LCD36 enters the prism as being refracted at the fourth surface 35 thereof,is reflected at the first reflecting surface on the second surface 33,is reflected at the third surface 34, then is reflected at the thirdreflecting surface on the second surface 33, exits out of the prism asbeing refracted at the first surface 32 thereof, and is imaged on animage surface 31.

[0199] In the case of FIG. 18, a prism P is provided with a firstsurface 32, a second surface 33, a third surface 34, and a fourthsurface 35. The first surface 32 is constructed and arranged as an exitsurface. The second surface 33 is constructed and arranged to provide,at different positions on the very same surface, a region that acts bothas an entrance surface and a second reflecting surface and a region thatacts as a fourth reflecting surface. The third surface 34 is constructedand arranged as a third reflecting surface. The fourth surface 35 isconstructed and arranged as a first reflecting surface. The prism P isconfigured so that light from a LCD 36 enters the prism as beingrefracted at the entrance surface thereof on the second surface 33, isreflected at the fourth surface 35, then is reflected at the secondreflecting surface on the second surface 33, is reflected at the thirdsurface 34, then is reflected at the fourth reflecting surface on thesecond surface 33, exits out of the prism as being refracted at thefirst surface 32 thereof, and is imaged on an image surface 31.

[0200] In the case of FIG. 19, a prism P is provided with a firstsurface 32, a second surface 33, and a third surface 34. The firstsurface 32 is constructed and arranged to act all in one as a firstreflecting surface, a third reflecting surface and an exit surface. Thesecond surface 33 is constructed and arranged as a fourth reflectingsurface. The third surface 34 is constructed and arranged to act both asan entrance surface and a second reflecting surface. The prism P isconfigured so that light from a LCD 36 enters the prism as beingrefracted at the entrance surface thereof on the third surface 34, isreflected at the first reflecting surface on the first surface 32, thenis reflected at the second reflecting surface on the third surface 34,is reflected at the third reflecting surface on the first surface 32, isreflected at the second surface 33, then exits out of the prism as beingrefracted at the exit surface thereof on the first surface 32, and isimaged on an image surface 31.

[0201] In the case of FIG. 20, a prism P is provided with a firstsurface 32, a second surface 33, and a third surface 34. The firstsurface 32 is constructed and arranged to act all in one as an entrancesurface, a second reflecting surface, a fourth reflecting surface and anexit surface. The second surface 33 is constructed and arranged as afifth reflecting surface. The third surface 34 is constructed andarranged to act both as a first reflecting surface and a thirdreflecting surface. The prism P is configured so that light from a LCD36 enters the prism as being refracted at the entrance surface thereofon the first surface 32, is reflected at the first reflecting surface onthe third surface 34, then is reflected at the second reflecting surfaceon the first surface 32, is reflected at the third reflecting surface onthe third surface 34, then is reflected at the fourth reflecting surfaceon the first surface 32, is reflected at the second surface 33, thenexits out of the prism as being refracted at the exit surface thereof onthe first surface 32, and is imaged on an image surface 31.

[0202] In the case of FIG. 21, a prism P is provided with a firstsurface 32, a second surface 33, a third surface 34, and a fourthsurface 35. The first surface 32 is constructed and arranged to act bothas a second reflecting surface and an exit surface. The second surface33 is constructed and arranged as a third reflecting surface. The thirdsurface 34 is constructed and arranged as a first reflecting surface.The fourth surface 35 is constructed and arranged as an entrancesurface. The prism P is configured so that light from a LCD 36 entersthe prism as being refracted at the fourth surface 35 thereof, isreflected at the third surface 34, then is reflected at the secondreflecting surface on the first surface 32, is reflected at the secondsurface 33, exits out of the prism as being refracted at the exitsurface thereof on the first surface 32, and is imaged on an imagesurface 31.

[0203] In the case of FIG. 22, a prism P includes a first prism P1 and asecond prism P2. The first prism P1 is provided with a first surface 32,a second surface 33, a third surface 34, and a fourth surface 35. Thefirst surface 32 is constructed and arranged to act both as a secondreflecting surface and an exit surface of the first prism P1. The secondsurface 33 is constructed and arranged as a third reflecting surface ofthe first prism P1. The third surface 34 is constructed and arranged asa first reflecting surface of the first prism P1. The fourth surface 35is constructed and arranged as an entrance surface of the first prismP1. The second prism P2 is provided with a first surface 41, a secondsurface 42 and a third surface 43. The first surface 41 is constructedand arranged to act both as a first reflecting surface and an exitsurface of the second prism P2. The second surface 42 is constructed andarranged as a second reflecting surface of the second prism P2. Thethird surface 43 is constructed and arranged as an entrance surface ofthe second prism P2.

[0204] The prism P is configured so that light from a LCD 36 enters thesecond prism P2 as being refracted at the third surface 43 thereof, isreflected at the first reflecting surface on the third surface 43, isreflected at the second surface 42, then exits out of the prism as beingrefracted at the first surface 41 thereof, enters the prism P1 as beingrefracted at the fourth surface 35 thereof, is reflected at the thirdsurface 34, then is reflected at the second reflecting surface on thefirst surface 32, is reflected at the second surface 33, then exits outof the prism as being refracted at the exit surface thereof on the firstsurface 32, and is imaged on an image surface 31.

[0205] In the case of FIG. 23, a prism P includes a first prism P1 and asecond prism P2. The first prism P1 is provided with a first surface 32,a second surface 33, a third surface 34, and a fourth surface 35. Thefirst surface 32 is constructed and arranged to act both as a secondreflecting surface and an exit surface of the first prism P1. The secondsurface 33 is constructed and arranged as a third reflecting surface ofthe first prism P1. The third surface 34 is constructed and arranged asa first reflecting surface of the first prism P1. The fourth surface 35is constructed and arranged as an entrance surface of the first prismP1. The second prism P2 is provided with a first surface 41, a secondsurface 42, a third surface 43 and a fourth surface 44. The firstsurface 41 is constructed and arranged as an exit surface of the secondprism P2. The second surface 42 is constructed and arranged as a secondreflecting surface of the second prism P2. The third surface 43 isconstructed and arranged as a first reflecting surface of the secondprism P2. The fourth surface 44 is constructed and arranged as anentrance surface of the second prism P2.

[0206] The prism P is configured so that light from a LCD 36 enters thesecond prism P2 as being refracted at the fourth surface 44 thereof, isreflected at the third surface 43, is reflected at the second surface42, exits out of the prism as being refracted at the first surface 41thereof, then enters the first prism P1 as being refracted at the fourthsurface 35 thereof, is reflected at the third surface 34, then isreflected at the second reflecting surface on the first surface 32, isreflected at the second surface 33, then exits out of the prism as beingrefracted at the exit surface thereof on the first surface 32, and isimaged on an image surface 31.

[0207] In the case of FIG. 24, a prism P includes a first prism P1 and asecond prism P2. The first prism P1 is provided with a first surface 32,a second surface 33, a third surface 34, and a fourth surface 35. Thefirst surface 32 is constructed and arranged to act both as a secondreflecting surface and an exit surface of the first prism P1. The secondsurface 33 is constructed and arranged as a third reflecting surface ofthe first prism P1. The third surface 34 is constructed and arranged asa first reflecting surface of the first prism P1. The fourth surface 35is constructed and arranged as an entrance surface of the first prismP1. The second prism P2 is provided with a first surface 41, a secondsurface 42, a third surface 43 and a fourth surface 44. The firstsurface 41 is constructed and arranged as an exit surface of the secondprism P2. The second surface 42 is constructed and arranged as a secondreflecting surface of the second prism P2. The third surface 43 isconstructed and arranged as a first reflecting surface of the secondprism P2. The fourth surface 44 is constructed and arranged as anentrance surface of the second prism P2.

[0208] The prism P is configured so that light from a LCD 36 enters theprism P2 as being refracted at the fourth surface 44 thereof, isreflected at the third surface 43, is reflected at the second surface42, exits out of the prism as being refracted at the first surface 41thereof, then enters the first prism P1 as being refracted at the fourthsurface 35 thereof, is reflected at the third surface 34, then isreflected at the second reflecting surface on the first surface 32, isreflected at the second surface 33, then exits out of the prism as beingrefracted at the exit surface thereof on the first surface 32, and isimaged on an image surface 31. The prism of FIG. 15 and the prism ofFIG. 16 show the following difference in configuration. Regarding thepath between the third surface and the fourth surface and the pathbetween the first surface and the second surface in the second prism P2,they do not intersect each other according to FIG. 23, while they dointersect each other according to FIG. 24.

[0209] Next, descriptions will be made of the modes in which theabove-described image observation optical system according to thepresent invention is reduced into realization in an image displayapparatus.

[0210] As an example, a head-mount type binocular image displayapparatus is explained in reference to FIG. 25 and FIG. 26. As shown inFIG. 26, this apparatus is configured to use the observation opticalsystem according to the present invention as an eyepiece optical system100 provided with an image display element 1. A pair of such eyepieceoptical systems 100 are provided and held spaced away from each other bythe interpupillary distance, to form a stationary-type or portable-typeimage display apparatus such as a head-mount type image displayapparatus for binocular observation.

[0211] The above-described observation optical system is used in a mainframe 102 of the image display apparatus as an eyepiece optical system100. A pair of such eyepiece optical systems 100 are provided as leftand right systems. Image display elements 1 constructed of liquidcrystal display elements are disposed on the respective image surfacesof the optical systems. As shown in FIG. 25, side-head frames 103 arecoupled to the main frame 102 on the lateral sides thereof so as to holdthe main frame 102 in front of the eyes of the observer. A cover member91 is disposed between the exit pupil of the eyepiece optical system 100and the first optical member 2. The cover member 91 may be any one of aplane parallel plate, a positive lens and a negative lens. It may beconstructed of a spectacle lens.

[0212] Also, each of the side-head frame 103 is equipped with a speaker104 so that the observer can enjoy stereophony, in addition to theimage. The main frame 102 provided with the speakers 104 as describedabove is connected with a player unit 106 for a portable video cassetteor the like via a video/audio transmission cord 105. The observer canenjoy image and sound upon holding the player unit 106 to an arbitraryposition, for example to her or his waist belt position, as shown in thedrawing. In FIG. 25, the reference numeral 107 represents a controlsection including a switch, a volume control etc. of the player unit106. Electronic devices such as video processing and audio processingcircuits are built in the main frame 102.

[0213] The end of the cord 105 may be formed as a jack to be plugged inan existing video deck etc. Also, the cord 105 may be connected with aTV tuner, which receives broadcasting waves, for observation of TVprograms, or may be connected with a computer to receive images ofcomputer graphics or text messages. Alternatively, the apparatus may beprovided with an antenna for receiving external signals carried by radiowaves, for the purpose of removing the cord, which is obstructive.

[0214] Also, as shown in FIG. 27, the observation optical systemaccording to the present invention may be applied to a head-mount typemonocular image display apparatus, which is designed so that an eyepieceoptical system is held in front of either eye (in the drawing, in frontof the left eye) of an observer. In this configuration, a main frame 102which is provided with a set including an eyepiece optical system 100and an image display element 5 is mounted on a front frame 108 at aposition in front of the corresponding eye. Side-head frames 103 shownin the figure are coupled to the front frame 108 on the lateral sidesthereof so as to hold the main frame 102 in front of the odd eye of theobserver. Other features are similar to those of the foregoing binocularconfigurations shown in FIGS. 25, 26 and thus explanation about them isomitted here.

[0215] Next, in reference to FIG. 28, a desirable arrangement incombining a diffraction element and a prism according to the presentinvention is explained. In the drawing, a decentered prism P correspondsto the prism included in the observation optical system of the presentinvention. In the case where a surface C of the diffraction element isshaped quadrangular as shown in the drawing, it is desirable, forbeautiful image forming, to make arrangement so that a plane of symmetryD of a plane-symmetric free curved surface on the decentered prism P isparallel to at least one side of the quadrangular surface C of thediffraction element.

[0216] Furthermore, if the surface C of the diffraction element forms aregular square or a rectangle with all of its interior angles beingsubstantially 90°, it is desirable to make arrangement so that the planeof symmetry D of the plane-symmetric free curved surface is parallel totwo opposite sides of the surface C and that the plane of symmetry Dcoincides with a horizontal or vertical plane of symmetry of the surfaceC of the diffraction element. Such an arrangement facilitates assemblyaccuracy and thus is effective for mass production.

[0217] Furthermore, if a plurality or all of optical surfacesconstituting the decentered prism P such as the first surface, thesecond surface, and the third surface are plane-symmetric free curvedsurfaces, it is desirable, in view of design convenience and inaberration performance also, to make arrangement so that the planes ofsymmetry of all of the plane-symmetric surfaces are arranged on thecommon plane D. It is also desirable to satisfy the above-mentionedrelationship between the plane of symmetry D and a plane of symmetry ofpower of HOE of the diffraction element.

[0218] As discussed above, according to the present invention, it ispossible to provide an image display apparatus which is reduced inweight while maintaining a good image quality.

What is claimed is:
 1. An image display apparatus comprising: an imagedisplay element; and an observation optical system which forms an exitpupil for observation of an image displayed on said image displayelement, wherein said observation optical system has at least onesurface that has a lens function, and the following condition issatisfied: 0.1<P·PD·ZD<5 where P is a pixel pitch (in μm) of said imagedisplay element, PD is a diameter (in mm) of said exit pupil, and ZD isa distance (in mm) from a display surface of said image display elementand a first surface which has a lens function.
 2. An image displayapparatus comprising: an image display element; and an observationoptical system which forms an exit pupil for observation of an imagedisplayed on said image display element, wherein said observationoptical system comprises at least one diffraction element that is givena lens function by diffraction effect and an optical member thatsatisfies the following condition: a<90 where a is a transmittance (in%) for a wavelength range of 450 nm-650 nm.
 3. An image displayapparatus comprising: an image display element; and an observationoptical system which forms an exit pupil for observation of an imagedisplayed on said image display element, wherein said observationoptical system comprises: a first unit that comprises at least one prismmember with a positive refracting power; and a second unit, and saidfirst unit is constructed to be movable for alignment of optical axes.4. An image display apparatus comprising: an image display element; andan observation optical system which forms an exit pupil for observationof an image displayed on said image display element, wherein saidobservation optical system comprises: a first unit that comprises atleast one prism member with a positive refracting power; and a secondunit with a positive refracting power, a primary image surface is formedbetween said first unit and said second unit, and the followingcondition is satisfied: 0.1<P·PD·ZDD<5 where P is a pixel pitch (in μm)of said image display element, PD is a diameter (in mm) of the exitpupil, and ZDD is a distance (in mm) along an optical axis from theprimary image surface to an optical element that is located closest tothe primary image surface.
 5. An image display apparatus comprising: animage display element; and an observation optical system which forms anexit pupil for observation of an image displayed on said image displayelement, wherein said observation optical system has a positiverefracting power, and the following condition is satisfied: 0.02×10⁻²<α·P<2×10⁻² where α is a field angle (in rad.) of said observationoptical system, and P is a pixel pitch (in μm) of said image displayelement.
 6. An image display apparatus comprising: an image displayelement; an observation optical system which forms an exit pupil forobservation of an image displayed on said image display element; and aclip section, wherein said observation optical system has a positiverefracting power, and a frame member provided with said observationoptical system is integrally formed with said clip section.
 7. An imagedisplay apparatus according to claim 1, wherein said observation opticalsystem comprises: a first optical member comprising a first surface thathas an action of reflecting bundles of rays from said image displayelement; and a second optical member having an action of furtherreflecting the bundles of rays reflected from said first surface, andwherein a space between said first optical member and said secondoptical member is filled with gas.
 8. An image display apparatusaccording to claim 1, wherein at least one surface of said observationoptical system is constructed of a diffraction element which is given alens function by diffraction effect.
 9. An image display apparatusaccording to claim 1, wherein the following condition is satisfied:0.5<P·PD·ZD<2.
 10. An image display apparatus according to claim 1,wherein at least one surface of said observation optical system has acurved surface shape to exert a power on bundles of rays, and saidcurved surface shape is configured as a rotationally asymmetric surfaceshape to compensate aberrations generated by decentering.
 11. An imagedisplay apparatus according to claim 2, wherein at least one anotheroptical element disposed between the exit pupil and said optical memberis a diffraction element.
 12. An image display apparatus according toclaim 2, wherein said observation optical system comprises at least oneprism member, and said prism member has an entrance surface via whichbundles of rays emergent from said image display element enter saidprism member, at least one reflecting surface which reflects the bundlesof rays inside said prism member, and an exit surface via which thebundles of rays exit out of said prism member, and wherein said at leastone reflecting surface has a curved surface shape to exert a power onbundles of rays, and said curved surface shape is configured as arotationally asymmetric surface shape to compensate aberrationsgenerated by decentering.
 13. An image display apparatus according toclaim 2, wherein said optical member satisfies the condition: a<50. 14.An image display apparatus according to claim 3, wherein at least onesurface in said observation optical system is configured as an opticalelement that is given a lens function by diffraction effect.
 15. Animage display apparatus according to claim 3, wherein said at least oneprism member in said observation optical system has an entrance surfacevia which bundles of rays emergent from said image display element entersaid prism member, at least one reflecting surface which reflects thebundles of rays inside said prism member, and an exit surface via whichthe bundles of rays exit out of said prism member, and wherein said atleast one reflecting surface has a curved surface shape to exert a poweron the bundles of rays, and the curved surface shape is configured as arotationally asymmetric surface shape to compensate aberrationsgenerated by decentering.
 16. An image display apparatus according toclaim 3, wherein said second unit has a positive refracting power. 17.An image display apparatus according to claim 3, wherein, for alignmentof an optical axis of said image display element, an optical axis ofsaid first unit and an optical axis of said second unit, said first unitis adjusted so that, upon a first pinhole being arranged on an exitsurface side of said first unit and aligned with the optical axis ofsaid first unit and a second pinhole being arranged on an exit surfaceside of said second unit and aligned with the optical axis of saidsecond unit, and then upon a central portion of said image displayelement being made to flash as a point light source, said point lightsource is observable through said first pinhole and said second pinhole.18. An image display apparatus according to claim 3, wherein aphotographing optical system is disposed on an exit pupil side so as tophotograph an image displayed on said LCD.
 19. An image displayapparatus according to claim 3, wherein said first unit is fixed withadhesive after alignment of the optical axes.
 20. An image displayapparatus according to claim 4, wherein a member that has an action ofinterrupting stray light from said image display element is disposedbetween said image display element and said exit pupil.
 21. An imagedisplay apparatus according to claim 4, wherein at least one flare stopis disposed between said first unit and said second unit.
 22. An imagedisplay apparatus according to claim 4, wherein at least one prismmember of said observation optical system has an entrance surface viawhich bundles of rays emergent from said image display element entersaid prism member, at least one reflecting surface which reflects thebundles of rays inside said prism member, and an exit surface via whichthe bundles of rays exit out of said prism member, and wherein said atleast one reflecting surface has a curved surface shape to exert a poweron bundles of rays, and said curved surface shape is configured as arotationally asymmetric surface shape to compensate aberrationsgenerated by decentering.
 23. An image display apparatus according toclaim 4, whrein the following condition is satisfied: 0.5<P·D·ZDD<2. 24.An image display apparatus according to claim 5, wherein saidobservation optical system comprises a first unit with a positiverefracting power and a second unit.
 25. An image display apparatusaccording to claim 24, wherein said first unit comprises at least oneprism member having a positive refracting power, and said prism memberhas an entrance surface via which bundles of rays emergent from saidimage display element enter said prism member, at least one reflectingsurface which reflects the bundles of rays inside said prism member, andan exit surface via which the bundles of rays exit out of said prismmember, and wherein said at least one reflecting surface has a curvedsurface shape to exert a power on bundles of rays, and said curvedsurface shape is configured as a rotationally asymmetric surface shapeto compensate aberrations generated by decentering.
 26. An image displayapparatus according to claim 24, wherein said second unit has a positiverefracting power.
 27. An image display apparatus according to claim 5,wherein at least one surface in said observation optical system is anoptical element that is given a lens function by diffraction effect. 28.An image display apparatus according to claim 5, wherein the followingcondition is satisfied: 0.05<P·LD<2 where LD is a distance (in mm) takenalong an image center between a last surface of said observation opticalsystem and the exit pupil.
 29. An image display apparatus according toclaim 5, wherein the following condition is satisfied: 0.1×10⁻²<α·P<1×10⁻².
 30. An image display apparatus according to claim 5,wherein the following condition is satisfied: 0.1<P·LD<1 where LD is adistance (in mm) between a last surface of said observation opticalsystem and the exit pupil taken along an image center.
 31. An imagedisplay apparatus according to claim 6, wherein said observation opticalsystem comprises a first unit having a positive refracting power and asecond unit.
 32. An image display apparatus according to claim 31,wherein said first unit comprises at least one prism member having apositive refracting power, and said prism member has an entrance surfacevia which bundles of rays emergent from said image display element entersaid prism member, at least one reflecting surface which reflects thebundles of rays inside said prism member, and an exit surface via whichthe bundles of rays exit out of said prism member, and wherein said atleast one reflecting surface has a curved surface shape to exert a poweron bundles of rays, and said curved surface shape is configured as arotationally asymmetric surface shape to compensate aberrationsgenerated by decentering.
 33. An image display apparatus according toclaim 32, wherein said second unit has a positive refracting power. 34.An image display apparatus according to claim 32, wherein at least onesurface in said observation optical system is a diffraction opticalelement.